cool! (about the cat!) well, how my nick here came about is that my usual nick is bijou. But some ingrate g got to my nick first at Renderosity so I had to choose a different one... so I chose bijouchat... as I am always called a cat by my boyfriend due to my rather feline nature. g anyway, about the formula, I suck at math, though I have two mathematics reference books to help with these things when I need it. (and that happens more often than you might think when doing shaders lol) But I do know a few things, and I know that in gemstones, the dispersion aka iridescence (and the patterns caused by refraction for that matter) are related to the refractive index. I use a cad program to help me with the correct angles for light refraction before the gemstone model ever enters Carrara. Carrara does refraction very well (helped along with the Fresnel shader from Lost Horizon though!), as is shown in my blue Hope model... all the patterns in that render are caused by refraction, none are postworked. Was a heck of a job getting the model to be shaped as much as possible like the real stone to pull that off. its important to note, that violet refracts more than red. So changing the colour to reflect more of the blues/violets and a little less of red would be more accurate regarding gemstone dispersion. I guess I better hit those pages about optics again... lol. Boy, gemstones have me running back to the science books all the time in the name of art g I'll be sure to show you what I come up with!